Nowadays, photovoltaic systems enjoy wide recognition in our society, as they are currently used more and more frequently. They are commonly facilities formed by a set of photovoltaic panels, an electronic converter that conditions the energy produced by the photovoltaic panels for its subsequent use and a series of protections and ducting inherent in any electrical installation.
A photovoltaic module or panel is an electrical device that converts the electromagnetic energy of light into electrical energy. This device generates a direct current according to the irradiance that falls on it and the voltage set between its terminals. The characteristic V-I (voltage-intensity) relationship of the photovoltaic module follows a curve in which three characteristic points can be highlighted, as shown in FIG. 1:                Voc-module: the open circuit voltage generated by the photovoltaic module when the current that flows through it is zero.        Isc-module: the short-circuit current generated by the photovoltaic module when the voltage between its terminals is zero.        Vmpp-module and Impp-module: the voltage and current points where the photovoltaic module generates the maximum power.        Pmpp-module: the maximum power generated by the photovoltaic module resulting from the following formula:Pmpp-module=Vmpp-module·Impp-module         
To increase the power generated by the photovoltaic facility, photovoltaic modules may be associated by connecting them to each other in series or in parallel.
The connection of one or more photovoltaic modules in series increases the power of the system by increasing the voltage of the system and keeping the current the same. This association in series of at least one photovoltaic panel is known as a series or string. Thus, if n panels are associated in series, the V-I curve resulting from this association will have as characteristic points:Voc-string=n·Voc-module Isc-string=Isc-module Pmpp-string=n·Vmpp-module·Impp-module 
Where:                Voc-string: the open circuit voltage generated by the string when the current that flows through it is zero.        Isc-string: the short-circuit current generated by the string when the voltage between its terminals is zero.        Pmpp-string: the maximum power generated by the string.        
Another alternative to increase the power is the connection in parallel; in this case, the voltage of the system is kept unchanged and the current increased. The parallel connection of one or several strings is known as a photovoltaic generator. If m strings (strings of n modules) are connected in parallel, the generator formed has a V-I curve with the following characteristics points:Voc-generator=n·Voc-module Isc-generator=m·Isc-module Pmpp-generator=n·m·Vmpp-module·Impp-module 
Where:                Voc-generator: the open circuit voltage generated by the generator when the current that flows through it is zero.        Isc-generator: the short-circuit current generated by the generator when the voltage between its terminals is zero.        Pmpp-generator: the maximum power generated by the generator.        
The photovoltaic field of the facility is obtained from the association in parallel of several of the photovoltaic generators described in the foregoing paragraph, together with their command, monitoring and protection devices (M11, . . . , Mpm) (FIG. 2).
Break and/or protection elements (M1-Mp) are often included in series with every photovoltaic generator.
Break elements (switches, disconnectors, etc.) allow switching off the current of the generators and are very useful for handling and maintenance operations of the facility. Protection devices (magnetothermal circuit breakers, fuses, etc.) are used to protect generators from surge currents that may damage the facility. If during the parallel connection of the different photovoltaic generators (G1-Gp) that make up the facility, the polarity of one of them is inverted by connecting its positive pole to the negative pole of the other generators and its negative pole to the positive pole of the other generators, a reverse current is generated (FIG. 3). In this case, the break and/or protection devices (M1-Mp) are subjected to double the voltage of the generator. This loading may even destroy the break and/or protection devices and cause serious personal injuries and material damage in that they are usually dimensioned for the voltage and current of the generator.
In the current state of the art, there are different alternatives to deal with this problem.
A first alternative consists in the use of mechanical electrical-connection elements (connectors) that set a univocal connection between two terminals. Once a cable is bound to its connector by either welding, crimping or screwing-in, there will only exist one possible connection.
The union between cable and connector, however, is frequently carried out at the facility itself, which can bring about the incorrect connection of the cable to its corresponding connector.
Another alternative consists in dimensioning the break and/or protection system for double the voltage of the generator. This is an effective solution but also more expensive, heavy and bulky.
A third option from the state of the art involves preventing the flow of current of the reverse-connected generator by providing each generator with a series-connected diode dimensioned for 2 times the open circuit voltage of the generator (FIG. 4). In normal operation, however, this diode causes electrical losses owing to the voltage drop inherent to the conduction of the diode with the resulting decrease in the performance of the system.